Flow ranges for single stage centrifugal compressors are set by the stalling characteristics of the impeller and the diffuser. These characteristics are intrinsically controlled by the diffusion capability or attainable static pressure rise across the blades in the vane rows. Generally, although not always, vaned diffuser systems are utilized in centrifugal compressors because they provide maximum efficiency at high Mach numbers. The vanes of the impeller and the diffuser must be matched simultaneously at their peak efficiency flow conditions.
It has been determined that in a vaned diffuser wherein the vanes are stationary, the diffuser tends to be the flow controlling component. Its overall Mach number level and inlet blockage are higher than those of the impeller which operates over a large radial variation of Mach numbers from hub to shroud in a radial discharge compressor. The diffuser is required to accept an already partially diffused flow from the impeller with resulting non-uniform entrance conditions about the inlet to the diffuser. This further aggravates stalling sensitivity and curtails the compressor operating range. Consequently, stationary vane diffusers for centrifugal compressors have received considerable attention.
Attainment of a wide flow range requires that the impeller and the diffuser be capable of extended operation into their stalled or positive incidence regions to flows where static pressure rise attains a plateau, and compressor surge is eventually triggered. Stage surge is believed to stem from operation on an unstable portion of the overall compressor characteristic curve whereat the static pressure ratio increases with increasing flow.
One effective method of increasing compressor operating range is to provide sufficient impeller stability so that the downstream diffuser can operate slightly into its positive incidence zone even though the diffuser static pressure recovery versus flow characteristic exhibits a slope indicative of unstability. Conventionally, increased impeller stability is obtained by imparting an initial swirl of the incoming gas at the compressor inlet in the direction of compressor rotation. To provide such stability over a wide range of flows, so-called variable inlet guide vane geometry is frequently employed. See, for example, U.S. Pat. No. 4,428,714 issued Jan. 31, 1984 to Mowill. While such variable inlet guide vane structures work well for their intended purpose, the guide vanes are typically pivoted about an axis with a substantial radial component and as a consequence, rather complicated linkages are required in order to simultaneously rotate the various vanes in a uniform manner. Consequently, rotary compressors having variable inlet guide vane geometry are substantially more complex than those without it and accordingly more expensive.
The present invention is directed to providing a rotary compressor with a simplified inlet guide vane structure to avoid the complexity and expense of the prior art.